The intensive use of granulated water-soluble chemicals, such as fertilizers, micronutrients, pesticides and other plant-protection agents in agriculture, requires their rational use taking into account the specific needs of the plants. The unbalanced use of these chemicals observed at present brings about, besides extra economic cost, a reduction of the biological value of foodstuffs and feeds and creates the environmental contamination hazard.
The main cause behind all this is fast solution of the chemicals used and the consequent losses as well as, correspondingly, the application to the soil of excess quantities of such chemicals as compensation for the losses sustained.
An efficient way of increasing the effect of chemicals, in particular fertilizers and chemical plant-protection agents, with a view to increasing the productivity of crops and reducing losses and costs in connection with their application consists in regulated introduction of chemicals into the soil, i.e. the use of chemicals with slow, controlled, solubility.
Slow release of chemicals excludes the possibility of poisoning the plants in cases of overdose of chemicals and makes it possible to add simultaneously increased quantities of micro- and macronutrients calculated for long-term action.
Despite higher costs, the production of slow-solubility chemicals holds promising prospects due to the continuous increase of prices for conventional chemicals and the necessity to increase the effect of the target substances since they are the determining factor as far as uniform application of chemicals to the soil and their capability of ion-exchange reactions with the soil components are concerned.
The known production methods of slow-solubility chemicals may be conditionally divided into those chemical and those physical, based on the nature of the solubility reduction rate.
In the case of substances based on the chemical slow-down of solubility (e.g., fertilizers, sintered ferrites, hardly soluble salts, ion-exchange media, urea formaldehyde, isobutylen urea, oxamide, and others) what is actually changed is either the mechanism of the nutrients' transition to an assimilable form or their form as such.
However, despite the wide variety of forms (oxides, salts, complexonites, ionites) and a broad spectrum of initial raw materials (from reactive compounds to industrial wastes), the chemical methods are unable to ensure arbitrary regulation of the chemicals' solution rate over a wide scale.
The physical methods of slowing down solubility are preferable since, due to a change in structure, they influence the kinetics without modifying the chemical composition of the chemicals and of assimilable forms of fertilizers.
Among physically modified compositions, the most widespread are multilayer granules whose solubility rate is determined by the thickness and composition of the outer coating film. As often as not, coatings made of various polymers, elementary sulfur, drying oils, resins, etc. are used to produce such films.
Another type of film of this kind is represented by a water-insoluble powder of non-organic origin which forms as a result of the hardening of liquid film on the surface of granules, or due to the lamination on their surface of finely dispersed powder. The most frequently used coatings are made of sulfur, phosphogypsum, sinters, dolomite, phosphorite meal, and magnesite.
The solubility reduction may also be achieved by compacting powders using both mechanical and thermal methods. These methods are used to produce slowly soluble chemicals of various geometrical forms: balls, cylinders, round tablets, and other briquettes. Various methods are used for the production of such chemicals of which the most important are compaction, extrusion, and crystallization of oversaturated fusion solutions in special moulds.
The common drawback of these methods consists in the difficulty of maintaining the admissible temperature of the mixture without local overheating and fluidity reduction. Additional difficulties may be encountered in the course of the extrusion process while cooling and cutting the braid; difficulties may also arise due to the working spinnerets wear.
The feasibility of using various coatings and methods of their application is almost totally determined by their production cost, i.e. by the cost of materials and additional technological operations.
The common drawback of known slow-solubility chemicals is their high cost, which is substantially higher than that of conventional chemicals. This is due to high energy requirements in compacting and sintering, the reduction of equipment reliability because of adhesion in the process of film-formation, and the use of expensive reagents in the complexing and ion-exchange processes. As a consequence, the known methods of slowing down the solubility of chemicals require improvements aimed mainly at increasing their effectiveness and reducing costs.
Among physical methods of the modification of chemicals, the technology based on the encapsulation of granulated water-soluble chemicals using water-resistant thermoplastic matrices of various nature is the one more rarely used due to the insufficient effectiveness of known materials and the high cost of polymer products synthesized for the purpose.
The present invention relates to methods of physical modification of granular chemicals and to their products. The method of reducing the solubility of chemicals is discussed and evaluated here in more detail.